Combustion and Spectral Methods for Quantifying Carbon and Nitrogen Concentrations in Pacific Northwest Douglas-Fir Forest Soils

Traditional combustion methods for assessing soil carbon (C) and nitrogen (N) stocks are time consuming and expensive; visible and near-infrared (VNIR) methods offer a quick and inexpensive alternative for establishing soil C and N concentrations. We compared combustion and spectral methods for quantifying soil carbon and nitrogen concentrations. We sampled organic and mineral soil horizons in managed and old-growth Douglas-fir (Pseudotsuga menziesii) forests in western Oregon. We applied combustion methods to determine total soil carbon and nitrogen concentrations of these samples. We then applied VNIR methods to derive a reference spectral library for analyzing Pacific Northwest (PNW) andesitic forest soils. Our spectral analysis confirmed that visible range spectra (especially in the 650-750 nm range) are the most useful for distinguishing differences in sample soil organic matter content. Our results provide a robust calibration model for applying spectral analysis combined with Partial Least Square Regression (PLSR) to quantify carbon and nitrogen stocks in PNW Douglas-fir forest soils. Model validation resulted in R2 values ranging from 0.92 to 0.95 for C and from 0.73 to 0.84 for N.